Compared to iron and nickel base alloys, various titanium alloys have favorable combinations of high strength, toughness, corrosion resistance and strength-to-weight ratios which render them especially suitable for aircraft, aerospace and other high-performance applications at very low to moderately elevated temperatures. For example, titanium alloys which have been tailored to maximize strength efficiency and metallurgical stability at elevated temperatures, and which thus exhibit low creep rates and predictable stress rupture and low-cycle fatigue behavior, are increasingly being used as rotating components in gas turbine engines.
After processing, titanium alloys are generally classified microstructurally as alpha, near-alpha, alpha-beta or beta. The class of the alloy is principally determined by alloying elements which modify the alpha (close-packed hexagonal crystal structure) to beta (body-centered cubic crystal structure) allotropic transformation which occurs at about 885.degree. C. (1625.degree. F.) in unalloyed titanium. Alpha alloys, alloyed with such alpha stabilizers as aluminum, tin, or zirconium, contain no beta phase in the normally heat-treated condition. Near-alpha or supra-alpha alloys, which contain small additions of beta stabilizers, such as molybdenum or vanadium, in addition to the alpha stabilizers, form limited beta phase on heating and may appear microstructurally similar to alpha alloys. Alpha-beta alloys, which contain one or more alpha stabilizers or alpha-soluble elements plus one or more beta stabilizers, consist of alpha and retained or transformed beta. Beta alloys tend to retain the beta phase on initial cooling to room temperature, but generally precipitate secondary phases during heat treatment.
The three major steps in the production of titanium and titanium alloys are the reduction of titanium ore to a porous form of titanium called sponge; the melting of sponge including, if desired, reclaimed titanium scrap (revert) and alloying additions to form ingot; and the formation of finished shapes as by remelting and casting or by mechanically working the ingots first into general mill products such as billet, bar and plate by such primary fabrication processes as cogging and hot rolling and then into finished parts by such secondary fabrication processes as die forging and extrusion.
Since many elements, even in small amounts, can have major effects on the properties of titanium and titanium alloys in finished form, control of raw materials is extremely important in producing titanium and its alloys. For example, the elements carbon, nitrogen, oxygen, silicon and iron, commonly found as residual elements in sponge, must be held to acceptably low levels since those elements tend to raise the strength and lower the ductility of the final product. Carbon and nitrogen are particularly minimized to avoid embrittlement.
Control of the melting process is also critical to the structure, properties and performance of titanium and titanium-base alloys. Thus, most titanium and titanium alloy ingots are melted twice in an electric-arc furnace under vaccum by the process known as the double consumable-electrode vacuum-melting process. In this two-stage process, titanium sponge, revert and alloy additions are initially mechanically consolidated and then melted together to form ingot. Ingots from the first melt are then used as the consumable electrodes for second-stage melting. Processes other than consumable-electrode arc melting are used in some instances for first-stage melting of ingot for noncritical applications, but in any event the final stage of melting must be done by the consumable-electrode vacuum-arc process. Double melting is considered necessary for all critical applications to ensure an acceptable degree of homogeneity in the resulting product. Triple melting is used to achieve even better uniformity and to reduce oxygen-rich or nitrogen-rich inclusions in the microstructure to very low levels. Melting in a vacuum reduces the hydrogen content of titanium and essentially removes other volatiles, thus producing higher purity in the cast ingot.
Titanium and its alloys are prone to the formation of defects and imperfections and, despite the exercise of careful quality control measures during melting and fabrication, defects and imperfections are infrequently and sporadically found in ingot and finished product. A general cause of defects and imperfections is segregation in the ingot. It is conventional wisdom that segregation in titanium ingot is particularly detrimental and must be controlled because it leads to several different types of imperfections that cannot readily be eliminated either by homogenizing heat treatments or by combinations of heat treatment and primary mill processing.
Type I imperfections, usually called "high interstitial defects" or "hard alpha," are regions of interstitially stabilized alpha phase that have substantially higher hardness and lower ductility than the surrounding matrix material. These imperfections are also characterized by high local concentrations of one or more of the elements nitrogen, oxygen or carbon. Although type I imperfections sometimes are referred to as "low-density inclusions," they often are of higher density than is normal for the alloy. In addition to segregation in the ingot, type I defects may also be introduced during sponge manufacture (e.g., retort leaks and reaction imbalances), heat formulation and electrode fabrication (e.g., during welding to join electrode pieces) and during melting (e.g., furnace malfunctions and melt drop-ins).
Type II imperfections, sometimes called "high aluminum defects," are abnormally stabilized alpha-phase areas that may extend across several beta grains. Type II imperfections are caused by segregation of metallic alpha stabilizers, such as aluminum, contain an excessively high proportion of primary alpha and are slightly harder than the adjacent matrix. Sometimes, type II imperfections are accompanied by adjacent stringers of beta which are areas low in both aluminum and hardness. This condition is generally caused by the migration of alloy constituents having high vapor pressures into closed solidification pipe followed by incorporation into the microstructure as stringers during primary mill fabrication.
Type I and type II imperfections are not acceptable in aircraft-grade titanium and titanium alloys because they degrade critical design properties. Hard alpha inclusions, for instance, tend to cause premature low cycle fatigue (LCF) initiation. Hard alpha inclusions are particularly detrimental as they are infrequently and sporadically found in ingot and finished product despite the exercise of careful quality control measures during the melting and fabrication and since, prior to the invention of the invention set forth herein, there was no known method to render harmless "melted-in" hard alpha defects.
Beta flecks, another type of imperfection, are small regions of stabilized beta in material that has been processed in the alpha-beta region of the phase diagram and heat treated. In size, they are equal to or larger than prior beta grains. Beta flecks are either devoid of primary alpha or contain less than some specified minimum level of primary alpha. They are localized regions which are either abnormally high in beta-stabilizer content or abnormally low in alpha-stabilizer content. Beta flecks are attributed to microsegregation during solidification of ingots of alloys that contain strong beta stabilizers and are most often found in products made from large-diameter ingots. Beta flecks also may be found in beta-lean alloys such as Ti-6Al-4V that have been heated to a temperature near the beta transus during processing. Beta flecks are not considered harmful in alloys lean in beta stabilizers if they are to be used in the annealed condition. On the other hand, they constitute regions that incompletely respond to heat treatment, and for this reason microstructural standards have been established for allowable limits on beta flecks in various alpha-beta alloys. Beta flecks are more objectionable in beta-rich alpha-beta alloys than in leaner alloys.